OFDM technology is mainstream technology for implementing high-speed wireless data transmission. OFDM technology theory is transmitting high-speed data using a large number of subcarriers that are orthogonal, and data rates of the subcarriers are relatively low. Compared to a typical frequency division multiplexing system, orthogonality of a subcarrier in OFDM improves spectral efficiency of the system. In OFDM, the whole signal bandwidth is divided into a plurality of narrow subcarrier frequency bandwidths, and flat fading occurs when the subcarrier bandwidths are smaller than the channel bandwidth. In this way, compared to the monocarrier system, flat fading in OFDM can be implemented in a more straightforward manner. Currently, OFDM technology is successfully applied to asynchronous digital subscriber lines (ADSL), digital television broadcasts (DVB) and wireless ATM (WATM) systems.
In order to improve a spectrum utilization rate in wireless systems, adaptive and high spectrum utilization rate-transmission technology has been requested for high-speed wireless data transmission for fading channels. In fading channels, compared to fixed coding modulation, adaptive modulation/coding technology is capable of effectively improving a throughput and an error rate (BER) of a system. Here, the throughput referred to is the spectrum utilization rate of the system—that is, the amount of information transmitted within a unit spectrum bandwidth and a unit time. The basic concept of AMC technology is adaptively changing one or more types of transmission power, symbol transmission rate, coordinate size, coding rate and coding mechanism based on channel characteristics at the current point and, when channel conditions are good, transmitting a large amount of information to increase spectrum utilization rate, and, when channel conditions are poor, transmitting a small amount of information to ensure a certain receiving BER request.
Before introducing an AMC method in OFDM, first, channel characteristics in OFDM transmission will be introduced briefly.
FIG. 1 shows an example of OFDM channel characteristics.
Here, two horizontal axes respectively indicate OFDM symbols on the time domain and subcarrier numbers on the frequency domain, and the vertical axis indicates channel gains corresponding to OFDM symbols and subcarriers. OFDM channels fluctuates in both time domain and frequency domain as a result of time domain spreading and time domain spreading of channels in transmission.
As described above, the concept of AMC is to change modulation and coding parameters in transmission based on channel characteristics at this current time. With OFDM, adaptivity in this case refers to adaptivity at two domains of time domain and frequency domain. Currently, as an adaptive configuration, AMC in OFDM is divided into two, AMC based on subcarriers and AMC based on subbands. The AMC based on subcarriers referred to here refers to carrying out transmission using a modulation method and a coding method that are different per OFDM subcarrier taking each subcarrier as a minimum unit of adaptivity. However, AMC based on subcarriers is very difficult to be implemented, and, in addition, has the problem that feedback overhead is too large. Typically, it is difficult to implement an AMC method based on subcarriers in an actual system. As another adaptive configuration in OFDM, a subband configuration using independent coding, namely, a subband adaptive method of the related art is, relatively, typically used.
FIG. 2 shows subband adaptive modulation and coding of the related art.
In this configuration, all of the subcarriers on the OFDM frequency domain are divided into several subbands. Here, a subband indicates a subcarrier group comprised of subcarriers in neighboring positions on the frequency domain. For example, in FIG. 2, the total number of subbands is N. One modulation coding block is then formed by the same subbands at several (in the case of FIG. 2, M) neighboring OFDM symbols. In subband adaptivity of the related art of FIG. 2, the modulation coding blocks carry out estimation of coding modulation parameters based on the channel characteristics and carry out independent coding. The numbers within the coding modulation blocks of FIG. 2 denote the classification coding modulation parameters of encoded modulation blocks belong to.
Typically, coding modulation parameters corresponding to classifications for the coding modulation parameters are decided in initial stages of a system. For example, the relationships between a classification, coding parameter, and modulation parameter are shown in Table 1 as an example. The present invention is by no means limited to Table 1.
TABLE 1ClassificationCoding parameterModulation parameter0Not transmittedNot transmitted1½TurboBPSK2½TurboQPSK3¾TurboQPSK4⅔Turbo8PSK5¾Turbo16QAM6⅔Turbo64QAM
Next, a block view implementing a subband adaptive method of the related art is shown in OFDM in FIG. 3.
FIG. 3A and FIG. 3B are block views showing an OFDM-AMC system combining OFDM and AMC of the related art.
When communication between a communication apparatus of FIG. 3A (transmission side) and communication apparatus of FIG. 3B (receiving side) is assumed to be carried out, typical examples are given by a base station (AP) of FIG. 3A and a mobile terminal (UE) of FIG. 3B. Further, assume that an AMC mechanism is used in transmission from FIG. 3A to FIG. 3B.
On the transmission side of FIG. 3A, information bits to be transmitted first pass through adaptive modulation/coding section 301. The output serial modulation symbols then pass through serial/parallel converter (S/P) 302 and inverse fast Fourier transformer (IFFT) 303 so that symbols in the frequency band are converted to the time domain. The symbols then pass through parallel/serial converter (P/S) 304 and have guard intervals inserted by guard interval insertion section 305. The bits are then transmitted via antenna 306. On the receiving side in FIG. 3B, after receiving transmission signals transmitted from the transmission side via receiving antenna 316, guard interval removing section 315 removes the guard intervals inserted on the transmission side. Further, the signal then passes through serial/parallel converter (S/P) 314 and fast Fourier transformer (FFT) 313 to be converted from time domain to frequency domain symbols. The signals are then subjected to parallel/serial conversion processing by parallel/serial converter (P/S) 312 and are finally outputted by adaptive demodulating/decoding section 311 to obtain received data.
Adaptive transmission from the transmission side of FIG. 3A to the receiving side of FIG. 3B is implemented mainly by adaptive modulation/coding section 301 on the transmission side and adaptive demodulating/decoding section 311 on the receiving side. As described above, the meaning of adaptive modulation and coding is to adaptively adjust modulation and coding parameters on the transmission side based on channel characteristics at the current time and to carry out demodulation and decoding using parameters corresponding to the transmission side on the receiving side. In a typical system, adaptive parameters required by adaptive demodulating/decoding section 311 depend on feedback from the receiving side. Before transmitting each data block, the receiving side always first estimates transmission channel from the transmission side to the receiving side at the current time by channel estimating section 319, and obtains channel characteristics of the subcarriers of the OFDM. Based on these channel characteristics, the receiving side then decides modulation and coding parameters used for the OFDM subbands in the case of transmitting data from the transmission side at the current point by parameter selecting section 318. Parameters for adaptive modulation and coding at the subbands obtained by parameter selecting section 318 have two uses.
The first use is the use as a parameter for modulation and coding at each OFDM subband when the transmission side transmits data at the current time. After selecting modulation and coding parameters of the OFDM subbands, subband AMC parameter selecting section 318 on the receiving side then transmits these parameters back to the transmission side via a feedback path of receiving side parameter transmitting section 320, antenna 316 on the receiving side, antenna 306 on the transmission side, and parameter receiving/extracting section 307 on the transmission side. After extracting these parameters, the transmission side controls adaptive modulation/coding section 301 using AMC control section 308.
The second use is the use as a parameter when the receiving side carries out demodulation and decoding. In AMC transmission, the receiving side is able to obtain accurate information bits for the first time by carrying out demodulation and decoding of received data based on modulation and coding parameters that are always the same as in the transmitting side. Subband AMC parameter selecting section 318 obtains and then transmits AMC parameters to adaptive demodulating/decoding section 317, and adaptive demodulating/decoding section 317 temporarily saves the AMC parameter. The AMC parameter needs to be used in control of adaptive demodulating/decoding section 311 on the receiving side.
In FIG. 4A and FIG. 4B, module 309 of FIG. 3A and module 321 of FIG. 3B are segmented to describe a method for employing subband AMC in OFDM of the related art in a clearer manner.
FIG. 4A and FIG. 4B show configurations for implementing subband adaptive modulation/coding of the related art.
On the transmission side of FIG. 3A, adaptive modulating/coding section 301 is comprised of adaptive coding section 401, interleave section 402, and adaptive modulation section 403. Data outputted from adaptive modulating/coding section 301 is transmitted to inverse fast Fourier transformer (IFFT) 303 via serial/parallel converter (S/P) 302. Transmission side AMC control section 308 controls adaptive modulating/coding section 301 based on modulation and coding parameters for the subbands obtained from parameter receiving/extracting section 307 of FIG. 3A. In subband adaptivity of the related art, coding modulation is carried out independently for the OFDM subbands. Namely, all subbands have respective independent modulation and coding parameters. AMC control section 308 controls adaptive modulating/coding section 301 using the obtained coding parameters C and modulation parameters M for the subbands. Further, AMC control section 308 obtains the number of information bits transmitted at the subbands based on the coding parameters C and modulation parameters M, generates a corresponding interleave matrix IT as a result, and controls interleave section 402 of adaptive modulating/coding section 301. On the transmission side, after AMC, a serial data stream 404 is obtained. This contains data transmitted in the order of subband 1, 2, . . . N, with modulation and coding methods of (C1, M1), (C2, M2), . . . , (CN, MN), respectively. After this, this data is subjected to serial/parallel conversion and is then sequentially mapped to subbands corresponding to OFDM and is transmitted.
All of the AMC parameters necessary in transmission of the data blocks on the transmission side are fed back from the receiving side. Namely, before the transmission side transmits the data blocks, the receiving side first needs to select AMC parameters for use in the data blocks that the transmission side transmits In the procedure where the receiving side selects parameters first, channel estimation is carried out using the received signal. A method based on a pilot or blind channel estimation etc. may be given as a method of channel estimation. After this, channel estimation section 319 transmits channel characteristics of the obtained OFDM subcarriers to subband AMC parameter selecting section 318. Subband AMC parameter selecting section 318 first carries out analysis of the performance of the subbands in OFDM in this way, and selects AMC parameters appropriate for the respective subbands from the selected set of AMC parameters. AMC parameters obtained in this way are then transmitted back to the transmission side via a feedback channel, and are used in actual adaptive modulation and coding operations when the transmission side carries out transmission, and also used at adaptive demodulating/decoding control section 409 on the receiving side. At the same time, taking time delay into consideration, parameter storing section 410 is necessary for storing parameters obtained at the current time. Adaptive demodulating/decoding section 311 on the receiving side is comprised of adaptive decoding section 408, deinterleaving section 407 and adaptive decoding section 406.
Compared to subcarrier adaptivity, the adaptive method using independent coding of subbands of the related art shown in FIG. 3A to FIG. 4B is able to effectively reduce the difficulty of implementation of adaptivity and is able to effectively reduce feedback overhead of the system. However, even in this kind of method, there is the drawback that it is not possible to effectively utilize diversity performance between the subbands.
Diversity is an important method for improving wireless transmission quality. The diversity referred to here is generally described as technology where the transmitting side increases redundancy of information using a certain resource and modifies or attenuates redundant information on both of the receiving side and the transmission side independently as much as possible, and, the receiving side utilizes and synthesizes the information in a collective manner, thereby obtaining a certain system gain. To summarize, this is technology where transmission is carried out simultaneously by utilizing a plurality of paths, and deficiencies in certain paths on the receiving side is compensated for by other paths.
In addition to the foundation of an independent coding method using subbands in OFDM adaptive modulation and coding of the related art, the present application is to obtain a patent for a method for combining subbands using a certain method, assuming the subbands as a subband group, then carrying out joint coding for subband groups. With AMC methods of the related art, a parameter is selected and coding is carried out for each subband independently, and, the method of the present application therefore seems to run counter to the concept of AMC of the related art in appearance. However, this method adopts diversity between subbands and is therefore able to obtain a larger coding gain. Further, if selection of modulation coding parameters is carried out within subband groups using the method proposed here, loss in transmission throughput is not generated compared with the method of the related art. By combining both, the method for which the present application seek a patent promotes improvement of adaptive transmission performance in OFDM.